All flocks of chickens have a well-defined order known as the pecking order. A chicken at the top of the pecking order gets to do the things she wants by 'pushing the others around' a little, giving them a short, sharp peck if they don't submit to her, allowing her to go where she wants to go or access food and drink first.
It is thought that chickens inherited a pecking order from their ancestors the Red Jungle Fowl of Thailand, who had a pecking order so that when food was available, there are no fights over food, attracting attention to the flock by predators. This way, the strongest at the top of the pecking order got to eat first, ensuring their survival and the survival of future generations.
The bird at the top of the pecking order will have first access to water, food, the best roosting place and so on. The bird at the bottom of the pecking order has the least ‘rights' in the flock and will usually be the last to the food and will ‘skirt' around food that is scattered for them, nipping in to grab a beak full when possible. She (or sometimes he) has last rights to food and other resources.
The following clip, taken from the BBC documentary "The Private Life of... Chickens" shows the pecking order in action:
Once pecking order is established and birds are fully grown, it usually remains unchallenged unless one of the birds becomes stronger, healthier or matures and challenges the hen higher up the pecking order.
When introducing new chickens to the flock, the pecking order is upset and needs to be re-established. This is very stressful for the flock and it is usually best to keep birds of similar ages together to prevent younger birds getting bullied too much or older birds being constantly challenged for their position in the pecking order as younger birds mature.